Using Build 15121 from MS App Store (nice, btw!), first run, I set up a Special Tile > HTML with a Tap action of “Open URL” wherein “Open in browser” is unchecked.
For the URL, I inserted a known-good “Local endpoint” for an RM5.1 rule that toggles a light in the same room. On tapping the tile, the command was clearly executed, as the rule immediately ran as expected.
However, this also happened (viewing with Windows Chrome, both latest) with each tap of the Tile:
For the record (in case it matters), the unique endpoint URL is of the form: